1. Field of the invention
The present invention relates to tree harvesting devices. More particularly, the present invention relates to tree harvesting devices which can be advantageously mounted on a mobile vehicle such as a tractor.
2. Description of the prior art
A number of tractor mounted tree harvesting devices are currently available to the logging industry which can fell trees in a faster and safer manner than can be done by means of a hand-operated chain saw. An early example of a tractor mounted tree harvesting device is disclosed in U.S. Pat. No. 4,161,200 to Albright. In this apparatus, a tree to be harvested may be pulled into a tree cradle by a pair of grab arms that are mounted on the tree cradle. Mounted on the cradle are a number of unadjustable stationary knives that prevent the tree from slipping downwardly when grasped by the grab arms. A saddle member which included a pivotable chain saw bar was mounted to the cradle some distance beneath the grab arms and stationary blades for cutting the tree.
A problem in tree harvesting apparatuses typified by the Albright '200 device was that the stationary blades would fail to penetrate some irregularly shaped trees which were clamped to the cradle for cutting, thus allowing the tree to slip downwardly and damage the chain saw bar.
Other devices have since been developed to help hold the tree stationary with respect to the saw while it is being cut. For example, in U.S. Pat. No. 4,491,163 to Kurelek, a fixed blade having a tapered edge is provided to enter the kerf after the chain saw bar so that the chain saw bar does not ride against the tree while turning. However, as discussed in the Kurelek patent, the function of the plate is more to reduce the horsepower necessary to cut through the tree than for supporting the tree. In addition, the fixed blade in Kurelek failed to prevent horizontal motion from taking place between the tree and the cradle.
In U.S. Pat. No. 4,219,059 to Albright, a movable spike was provided to penetrate the tree stump beneath the chain saw bar to stabilize the cradle with respect to the tree prior to cutting. However, the apparatus had no means other than a pair of grab arms to secure the tree against downward movement with respect to the saw blade during cutting.
It is clear that there is a long and unfilled need in the prior art for a tractor mounted tree felling apparatus having an improved mechanism for preventing a tree from slipping downwardly and pinching or damaging the chain saw blade during cutting.